Nowadays, because of the gradual depletion of fossil energy reserves, it becomes increasingly important and indispensable to obtain energy from renewable power sources. Some renewable power source—for example solar power, geothermal power, etc.—can be harnessed with various efficiencies. The exploiting of wind energy seems to be promising, but in the case of wind energy, there are several factors, which restrict utilisation. With ground-installed wind power plants, a frequently occurring problem is the unsteady nature of the wind that makes difficult the use of this power source predictably and continuously. This problem is diminishing, if a wind power plant is located in the highest possible atmospheric layers, because at higher altitudes the wind is more balanced. Spreading of ground wind power plants therefore hits against significant difficulties, and hence other solutions are necessary for harnessing potentially useable wind energy at altitudes unavailable for ground power plants.
Due to the emerging needs, several approaches are known for soaring wind power plants. According to U.S. Pat. No. 7,821,149 B2 and U.S. Pat. No. 7,582,981 B1, the hovering of the structures in the air is ensured by a tank filled up with a gas lighter than air, by a kiting structure or, according to the latter document, by a helicopter. In the wind power plant according to these documents, power is generated by turbines running on a common shaft. The turbines rotate in opposite directions, and their power is controlled in a way that the torque exerted on the wind power plant is to be zero. The documents also describe a wind power plant system in which several wind power plants are connected in series by means of ropes, i.e. they are hovering in offset positions. It is a disadvantage of the wind power plant described in these documents that the lifting and hovering in some embodiments are ensured by means of passive devices, where time to time it may become necessary to refill the tanks by an appropriate type of gas, and also that in unflattering weather conditions the lifting and the hovering cannot be ensured merely by a kiting structure. In other embodiments, the lifting and the hovering are ensured by an active structure, for example a helicopter, in which case the hovering of the structure requires continuous power input, which is disadvantageous. A further disadvantage of the wind power plant described in these documents is that the torque acting on the structure is minimised by controlling the output provided by the turbines turning in opposite directions.
In US 2011/0057453 A1, U.S. Pat. No. 6,781,254 B2, U.S. Pat. No. 7,109,598 B2 and U.S. Pat. No. 7,183,663 B2 wind power plants are disclosed, by which tiltable rotors are fixed to several points on a frame. A kite is also secured to the frame to ensure the hovering of the structure. The direction of rotation of the rotors is determined in a way that the resultant torque acting on the structure is zero. In the course of lifting, the rotors ensure the lifting of the wind power plant. When the structure reaches an appropriate height, some of the rotors switch to generator mode and generate power, while the other rotors contribute to the hovering of the structure. It is a disadvantage of the wind power plant according to the documents that when the tilting of the rotors, i.e. their symmetric arrangement is discontinued, the resultant torque acting on the structure will not be zero.
In U.S. Pat. No. 4,285,481 a wind energy exploiting structure is disclosed where the hovering is ensured by a kiting aerofoil, to which turbines converting wind energy into electric power are adjoined. It is a disadvantage of this structure that due to the arrangement of the turbines, zero torque acting on the apparatus is not ensured and therefore the hovering is unstable.
In U.S. Pat. No. 7,317,261 B2 a wind power plant is disclosed, which consists of a plurality of in series connected hovering power generating units. A hovering wind power plant is disclosed in US 2003/0006615 A1. In US 2011/0025061 A1 a control system of a hovering wind power plant is disclosed.
In light of known approaches, a need has emerged to develop a power generating apparatus harnessing wind energy, which eliminates the disadvantages of the known solutions.